Influenza is usually a mild illness of the upper respiratory tract. Involvement of the lower respiratory tract, however, can lead to 4 types of clinical syndromes: physical signs of lower respiratory tract involvement without roentgenographic evidence of pneumonia, influenza complicated by bacterial pneumonia, primary influenza virus pneumonia, and combined influenzal and bacterial pneumonias.

Influenza virus infections are most severe in patients with certain preexisting conditions such as rheumatic heart disease, bronchopulmonary disease, impaired renal function, and diabetes mellitus. Infections can be more severe in elderly patients, pregnant females, and immunocompromised patients.

Influenza virus type A can produce serious illness during the first 2 years of life, with croup, bronchitis, and pneumonia being prominent.

Influenza A may also precipitate asthmatic attacks and produce chronic pulmonary complications in children.